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IC 91 ° 9 



Bureau of Mines Information Circular/1986 



Evaluating Ventilation Parameters 
of Three Coal Mine Gobs 



By R. J. Timko, F. N. Kissell, and E. D. Thimons 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9109 



Evaluating Ventilation Parameters 
of Three Coal Mine Gobs 



By R. J. Timko, F. N. Kissell, and E. D. Thimons 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 







Library of Congress Cataloging in Publication Data: 



Timko, Robert J 

Evaluating ventilation parameters of three coal mine gobs. 

(Information circular ; 9109) 

Bibliography: p. 16. 

Supt. of Docs, no.: I 28.27: 9109. 

1. Mine ventilation. 2. Mine gases. 3. Coal mines and mining- Safety measures. 4. Mine 
shafts. I. Kissell, Fred N. II. Thimons, Edward D. III. Title. IV. Series: Information circular 
(United States. Bureau of Mines) ; 9109. 



TN295.U4 



[TN301] 



622 s 



[622'.8] 



86-600132 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Mine A 3 

Background 3 

Borehole examination 3 

Results 4 

Conclusions 5 

Mine B 6 

Background 6 

First gob evaluation 7 

Second gob evaluation 9 

Gas analyses of left and right inby positions 10 

Conclusions 10 

Mine C 11 

Background 11 

Evaluation 13 

4 South 13 

3 South 13 

2 South 13 

Conclusions 14 

4 South 14 

3 South 14 

2 South 15 

Summary 15 

References 16 

ILLUSTRATIONS 

1. Ventilation schematic and sampling stations of Mine A gob 4 

2. Sealed gob and sampling stations of mine B 7 

3. Mine B seals with urethane foam seal outby on roof and ribs 3 

4. Standing water located outby Mine B seals 9 

5. Sealed gob and sampling stations of Mine C 12 

TABLES 

1 . Differential pressure change with depth 3 

2. SFg detection time and average gas velocity through the gob from the bore- 

hole to sampling stations 2-9 4 

3. Distances from borehole to western sampling points 6 

4. Gas concentrations behind seal A and seal D, Mine B 10 





UNIT OF MEASURE 


ABBREVIATIONS 


USED IN 


THIS REPORT 


ft 


foot 




in w.g 


;. inch, water gauge 


ft 3 


cubic foot 




min 


minute 


ft 3 /h 


cubic foot per hour 




ppb 


part per billion 


f t/min 


foot per minute 




ppm 


part per million 


ft 3 /min 


cubic foot per minute 


pet 


percent 


h 


hour 




psi 


pound per square inch 



EVALUATING VENTILATION PARAMETERS 
OF THREE COAL MINE GOBS 



By R. J. Timko, 1 F. N. Kissell, 2 and E. D. Thimons 3 



ABSTRACT 

The Bureau of Mines used sulfur hexafluoride (SFg) tracer gas to eval- 
uate the effectiveness of gob ventilation and/or sealing practices at 
three coal mines, each having different problems associated with their 
mined-out areas. The purpose of these ventilation studies was to bet- 
ter understand whether current techniques employed for ventilation or 
sealing are successful at minimizing the potential for gob fires and ex- 
plosions. The work performed at each mine is discussed: One is a long- 
wall operation that uses ventilation to carry off hazardous gases, and 
two are room-and-pillar operations, each with far different concerns, 
that seal off the gobs to isolate them from the main mine ventilation. 
In all cases employment of the Bureau's SFg tracer gas technique re- 
sulted in answers to questions raised by the mine operators concerning 
the effectiveness of their gob ventilation or sealing practices in 
preventing mine fires. 



1 Physical scientist. 
2 Research supervisor. 
^Supervisory physical scientist. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



Coal mining involves the fracturing and 
removal of a certain portion of the coal 
seam; a production goal is to remove the 
maximum amount of coal without compro- 
mising safety of the miners. High ex- 
traction techniques, such as longwalls, 
have become common. In room-and-pillar 
operations, retreat or second mining is 
frequently performed. This involves sys- 
tematic removal of coal pillars that 
remain after the primary mining cycle, 
which results in controlled caving of the 
strata above the mined-out area, or gob. 

To prevent the mixing of explosive con- 
centrations of methane (5 to 15 pet CH 4 
in air) and oxygen-rich mine air, mine 
operators may employ one of two tech- 
niques: (1) continue to ventilate the 
gob to keep the methane concentration 
low, or (2) seal off the section to iso- 
late it from the main mine ventilation. 
The Mine Safety and Health Administration 
(MSHA) has regulations or specifications 
regarding either technique. For ventila- 
tion, MSHA requires that sufficient air 
be used to carry off hazardous gases 
(5). 4 

MSHA's specifications for gob sealing 
require that seals be made of substantial 
and incombustible materials that are 
strong enough to prevent explosion propa- 
gation. Because of varying pressure dif- 
ferentials, mineral friability, and 
strata convergence, seals are never com- 
pletely airtight. Their purpose is to 
reduce the potential for combustion. On 
the one hand, they restrict the passage 
of mine air containing high quantities of 
oxygen into gobs that have a potential 
for spontaneous combustion; on the other 
hand, they reduce the passage of explo- 
sive gases liberated from the gob to the 
areas of the active mine. 

If ingassing of mine air to the gob 
occurs, fires can be started through 
spontaneous combustion, which usually 
begins slowly but can accelerate to haz- 
ardous proportions before detection. If 
outgassing of gob gas into active areas 
occurs, explosions can happen where 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report 



sufficient quantities of methane (5 to 15 
pet in air) are present. Even without 
the presence of methane, problems can oc- 
cur if blackdamp (C0 2 ) and/or whitedamp 
(CO) leak through permeable seals. 

This report gives an overview of in- 
vestigations at three different coal 
mines having very different gobs. 

1. At Mine A, which employs mainly 
longwall extraction techniques, it was 
necessary to ventilate the remaining gob. 
Research determined the permeability as 
well as average air velocities through 
the gob. 

2. At Mine B, which uses room- 
and-pillar extraction, the gobs were 
sealed. The operators had been taking 
gas samples from two seals in the same 
small gob. They were concerned because 
sample analyses did not agree. Research 
was performed to determine if continuity 
existed across the gob and to evaluate 
the performance of the seals. 

3. At Mine C, which also uses the 
room-and-pillar method, the mined-out 
gobs were sealed. The particular gob 
studied was very large, with three sepa- 
rate but interconnected sections, having 
a total of 18 seals separating it from 
the active mine. The coal seam pitched 
downward so that the methane concentra- 
tion was different behind each set of 
seals. These tests examined the poten- 
tial interconnection of the gob entries 
as well as the performance of the section 
seals. 

Sulfur hexafluoride (SF 6 ) was the 
tracer gas used in all gob evaluation 
work. SF 6 is a colorless, odorless, and 
tasteless gas having a threshold limit 
value-time weighted average of 1,000 ppm. 
This low toxicity means that workers can 
be repeatedly exposed to 1,000 ppm of SF 6 
for a normal 8-h day and 40-h week with- 
out encountering adverse effects (J_). 

SF 6 is stored in small, low-pressure 
(300-psi), metal lecture bottles. The 
gas has a volume of approximately 1.5 ft 3 
at atmospheric pressure. The Bureau has 
developed techniques for releasing and 
sampling SF 6 , as well as for reducing the 
data obtained (3"~4)« Typically, the SF 6 
is released, then sampled by puncturing 
air-evacuating sampling tubes, which are 



similar to tubes used for blood collec- concentrations are determined through 
tion. These tubes are then returned electron-capture gas chromatographic 
to the laboratory where actual gas analysis. 



MINE A 



BACKGROUND 



Mine A, located in southwestern Penn- 
sylvania, was required to ventilate, 
rather than seal, the gobs remaining 
after longwall mining. The gob area com- 
prised four longwall panels whose perim- 
eters contained entries to properly ven- 
tilate the gob. The longwall gob being 
evaluated was in the second developed 
panel, which was 490 ft wide and 3,900 ft 
long. Typical seam height in this mine 
was 60 in. 

At times, gob outgassing of methane in 
Mine A was a problem. Therefore, the 
mine had begun sinking boreholes into the 
longwall panels prior to mining. The 
topography above the mine made actual 
borehole placement difficult. The bore- 
hole in this gob was driven to the cen- 
terline of the gob, approximately 500 
ft from the eastern bleeder entries. 

The objective of this investigation was 
to measure gas velocities through the 
caved gob. This was accomplished by cap- 
ping the borehole and injecting SF6 down 
a tube through the borehole into the gob. 
Several underground sampling locations 
around the perimeter of the gob were set 
up to detect the SFg. The total time re- 
quired for the SF6 to traverse the gob 
and enter the ventilation airstream could 
be converted to an average velocity, ex- 
pressed in feet per minute. 

BOREHOLE EXAMINATION 

Prior to beginning the evaluation, the 
flame arrester and check valve were re- 
moved from the borehole. The borehole 
was then capped and 0.25-in-ID, semirigid 
tubing was lowered through the cap into 



the hole. The tubing served two pur- 
poses. First, it was used to release the 
SF6 just above the caved gob. Second, 
SFg samples were taken from the tubing 
at specific intervals during the test. 

Static pressure at the top of the bore- 
hole was 3.0 in w.g. The pressure dif- 
ference between the top of the borehole 
and the bottom of the tube was measured 
as the tubing was lowered down the hole. 
Two hoses were connected to a Dwyer 5 Mag- 
nehelic pressure gauge: one from the 
borehole top, and another from the fixed 
end of the tube descending the borehole. 
Table 1 shows that pressure differ- 
ence increased somewhat linearly from 
in w.g. at the borehole top to 0.08 
in w.g. at 500 ft. 

At 570 ft, or approximately 200 ft 
above the coal seam, an obstruction was 
encountered, and the pressure differen- 
tial began to climb rapidly. The tubing 
was then raised slightly to return the 
pressure differential to its original 
value (0.08 in w.g.). 

SFg was then injected through the tub- 
ing. To ensure that the SFg would be re- 
leased into the gob and not remain in the 
tubing, the tube was then purged with 
nitrogen. However, problems were en- 
countered with the nitrogen purge, and 
SFg release was not completed until ap- 
proximately 3.5 h after the release 
start. This was the official start time 
of the test. Sampling took place at 15- 
min intervals at nine underground bleeder 
entry stations at the gob perimeter, as 
well as at the borehole. 

-^Reference to specific manufacturers 
does not imply endorsement by the Bureau 
of Mines. 



TABLE 1. - Differential pressure change with depth 



Depth, ft: 



Pressure differential, 
in w.g. 



Depth, ft: 



Pressure differential, 
in w.g. 



Surface. 

100 

200 



'0.00 
.04 
.04 



300. 
400. 
500. 



0.04 
.08 
.08 



Actual values were limited by the resolution of the instrument. 



To determine initial results, samples 
taken during the first 3 h at the bore- 
hole bottom were analyzed immediately. 
Results showed that the SFg in the bore- 
hole was not diffusing very rapidly. 
Therefore, sampling frequencies were re- 
duced to one sample every 30 min, and the 
test time extended. 

Borehole samples were again analyzed on 
the following day to determine if SFg 
dispersion was increasing. After 27 h of 
sampling, SFg concentrations at the bore- 
hole bottom had been reduced from approx- 
imately 20,000 ppb to just above 100 ppb. 
Sampling continued, but at a reduced fre- 
quency of one sample per hour, for 1 week. 
(168 h). 

Figure 1 is a ventilation schematic of 
the gob being evaluated, showing bleeder 
entry sampling stations 1-9 and the bore- 
hole. Position A denotes the shortest 
two-dimensional path from the borehole to 
a bleeder entry. Also shown are approxi- 
mate two-dimensional distances from the 
borehole to each sampling station. 

Stations were divided into three 
groups: (a) no SFg detected (b) detec- 
tion within 3 h, and (c) detection be- 
tween 40 and 57 h. Only station 1 de- 
tected no SFg, probably because the 
borehole was between it and the fan. 
Stations 2-6 detected SFg within 3 h 
after gas release. Stations 7-9 first 
detected SFg between 40 and 57 h after 
the test began. Sampling was halted 
after 144 h. 




LEGEND 
■ Sampling station 



All distances approx ft 



FIGURE 1.— Ventilation schematic and sampling stations of 
Mine A gob. 



RESULTS 

Table 2 shows the approximate time from 
test start to detection of SFg at each 
station (except station 1) . Average gas 
velocity (V) was determined for both the 
last sampling time prior to gas detection 
and the first sampling for which SFg was 
detected at the station, according to the 
equation 

V = D/T, 

where V = average gas velocity, ft/min, 

D = distance from borehole to 
station, ft, 



and 



T = detection time, min. 



TABLE 2. - SFg detection time and average gas velocity through the 
gob from the borehole to sampling stations 2-9, Mine A 



SFg flow 


2-dimen- 
sional dis- 


Last sampling 


before detection 


1st sampling showing SFg 


from bore- 


Time , rain 


Gas velocity, 


Time, min 


Gas velocity, 


hole to — 


tance, ft 




ft/min 




ft/min 


East side: 












Station 2. . . 


800 


NAp 


NAp 


45 


17.78 


Station 3. . . 


2,000 


30 


66.67 


90 


22.22 


Station 4.. . 


2,100 


45 


46.67 


105 


20.00 


Station 5. . . 


3,600 


30 


120.00 


90 


40.00 


Station 6 . . . 


1 4,350 


105 


41.43 


165 


26.36 


West side: 












Station 7 . . . 


3,900 


2,395 


1.63 


3,397 


1.15 


Station 8. . . 


3,900 


2,430 


1.58 


2,741 


1.42 


Station 9. . . 


3,600 


2,725 


1.32 


3,420 


1.05 



-1 ' = ■ 

Indirect path, via station 7. 



Looking at only stations 7-9 (on the 
west side of the_gob) , one might assume 
that the overall V was no more than 1.63 
ft/min. However, the wide range in V at 
stations 2-6 (18 to 120 ft/min) indicated 
that the SF6 was not traveling directly 
to those stations through the gob but 
must have entered the bleeder entries at 
some point and was carried along the air- 
way to these sampling stations. If SFg 
was to take the shortest two-dimensional 
path to the bleeder entries (500 ft), 
it would enter the bleeder airstream at 
position A in figure 1. If V for sta- 
tions 7-9 had been realistic, travel time 
from the borehole to position A should 
have been approximately 6 h; but in ac- 
tual testing SF6 was detected at station 
2, just downstream from position A, after 
only 45 min. 

Bleeder entry air velocity was measured 
using an anemometer at 300 ft/min, and 
travel time from station 1 to station 6 
was as follows: 



From 


To 


Time, rain 






2.7 






4.7 






4.0 






8.3 






2.3 




22.0 



From this information, it was possible 
to approximate the gas velocity through 
the gob to position A, Since ventilation 
air traveled from position A to station 2 
in only 2 rain, then the maximum time for 
the gas to move through the gob to the 
bleeder entry (at position A) is 43 min; 
this results in a V through the gob of 
12.8 ft/min, which is a minimum figure. 
This is approximately 10 times the veloc- 
ity toward the west side of the gob 
(range 1.05 to 1.63 ft/min). The wide 
variation from the east to west sides of 
the gob may be due to several factors, 
including gob caving, proximity to the 
bleeder, borehole position, and atmos- 
pheric pressure differences in the sur- 
rounding bleeder entries. 

Once SFs entered the bleeder entries, 
it acquired entry velocity and moved 
toward the fan. SFg was detected at 
stations 2-6 within the same sampling 



interval, indicating that the gas took 
the shortest possible path from the bore- 
hole to point A. The SFg concentrations 
peaked rapidly, then reapproached zero 
asymptotically. No "second spike" or 
subsequent increase in SFg concentration 
was ever recorded at stations 2-6; such 
a change would have indicated a direct 
flow of SF6 from the borehole, through 
the gob, to that station and could have 
given researchers an average gas velocity 
throughout the entire gob. 

Assuming that the true gob velocity was 
12.8 ft/min, the ventilation paths to the 
western bleeder sampling stations (7-9)6 
may have been so tortuous that the air 
required a greater time to permeate the 
gob. In an attempt to place a third di- 
mension in the gob, the equation 
D = R x T was used where R was 12.8 
ft/min, T was the gas detection time 
(min), and D was the total distance (ft). 
From table 3, it is apparent that the 
distances derived were unrealistically 
large. This confirms the results of 
table 2. The velocities in the western 
side of the gob were significantly lower 
than those of the eastern side. 

CONCLUSIONS 

The objective was to determine the 
ability of ventilation air to adequately 
enter and ventilate a caved gob. The 
lone exhausting borehole was capped and a 
small-diameter tube placed down the hole 
from the surface to a position just above 
the actual caving. SF6, followed by an 
inert purge gas, was released into the 
gob. Nine underground bleeder evaluation 
stations, in addition to the borehole 
location, were sampled for SF6 over 
a total of 6 days. SF6 was detected 
in less than 3 h at the eastern sam- 
pling points (nearer the borehole) , 
whereas detection required 2 days at 
the western sampling points farthest from 
the borehole. 

^As shown in figure 1 , the path of SFg 
to station 6 was not through the gob 
(hence, not via station 7), and could not 
have been through the bleeder entry from 
station 5, but was likely through a 
parallel bleeder entry (shown) . 



TABLE 3. - Distances from borehole to western 
sampling points 





Gas detection 
time, min 


Distance from 


borehole, ft 




2-dimensional 


3-dimensional 


Station 7 : 










2,395 


3,900 


30,656 




3,397 


3,900 


43,481 


Station 8: 










2,430 


3,850 


31,104 




2,741 


3,850 


35,084 


Station 9; 










2,725 


4,000 


34,880 




3,420 


4,000 


43,776 



The following conclusions were derived 
regarding the permeability of the caved 
gob. This gob was permeable, since SF6 
was detected after a short sampling time. 
The location of the borehole tended to 
enhance gas escape toward the eastern 
working areas even though mine atmos- 
pheric pressures were lower on the wes- 
tern side of the gob. If air quantities 
from the respective entries remained the 
same when the borehole was reopened, then 
one side of the gob was being ventilated 
more efficiently than the other. 

The location of the borehole appeared 
to be a critical factor in the perform- 
ance of this gob. Multiple boreholes in 
each panel, or possibly a more strategic 



location of the single borehole, would 
improve gob ventilation. 

If multiple boreholes are not feasible, 
an alternative would be to ensure that a 
measurable pressure differential existed 
across the gob, and that a perceptible 
quantity of air was actually flowing into 
the gob. Since air movement is directly 
related to pressure differential, if a 
differential exists across the gob, some 
air movement will take place. There was 
a pressure differential across the gob in 
Mine A. This, in conjunction with the 
borehole, was apparently sufficient to 
minimize problems associated with gob 
ventilation. 



MINE B 



BACKGROUND 



Research at Mine B, an underground coal 
mine in western Colorado, was performed 
on a sealed gob. The coal in this par- 
ticular seam is prone to spontaneous com- 
bustion. The gob was in a section driven 
90° westward from the mains for approxi- 
mately 700 ft before executing a 90° 
right turn and continuing development 
parallel to the main entries. Initially, 
five entries were driven; two intakes, 
two returns, and one belt entry. After 
the 90° right turn, only four entries 
were driven for just under 400 ft, after 
which the section was expanded to seven 
entries (fig. 2) . 



Approximately 700 ft past the 90° right 
turn, a vertical fault was encountered. 
Due to the extent of the vertical dis- 
placement, the development was halted, 
and the section was retreat-pillar-mined 
back to the four-entry area. Seals were 
then installed in each of the four en- 
tries. Because of the quantity of over- 
burden and the resultant strata conver- 
gence, each seal had to be constructed of 
wood blocks. These pine blocks were 
built to an approximate thickness of 3 
ft. Sprayed-on rigid ure thane foam was 
applied to the seal faces as well as to 
the ribs and roof for 10 to 15 ft outby 
(fig. 3). Sampling ports in seals A and 



a oa 




LEGEND 

Intake air SAMPLING POINTS 
/ Left return 

2 to £'Left seal inby 
(via tube) 

3 to 5' Left sealoutby 
(via tube) 

4 Right seal outby 
5 to 5' Right seal inby 
(via tube) 



— ■— Return air 
n Regulator 
=0= Door 
— C — Curtain 
=== Stopping 
=== Seal 

® Overcast 
CO Water 

Semirigid 

sampling 
tube 



Right return 



FIGURE 2.— Sealed gob and sampling stations of Mine B. 



D made it possible to take gob samples 
remotely via tubing. 

The objective of these tests was to 
determine why the oxygen concentration at 
sampling point 5 was much higher than ex- 
pected. This was a concern because of 
the potential for spontaneous combustion. 
SF6 was released directly into seal D 
through the sampling port. To determine 
seal performance and possible migration 
of SF6, gob air samples were taken behind 
the seals from the sampling ports at 
points 2 and 5. Additionally, SFg sam- 
ples were taken from positions just outby 
the left and right seals (points 3 and 
4, respectively), and from the left 
and right returns (points 1 and 6, 
respectively). 

Prior to this research, another SF6 
tracer gas survey at the same location 
had been conducted by an MSHA technical 
support group. MSHA found that detect- 
able SF6 was neither permeating the large 
vertical fault that had previously halted 
mining in the section, nor entering a 
nearby return entry. Because of the 
proximity of the main return to the 
sealed section, a large pressure 
differential existed between the two. 



Had SF6 been detected in the return, it 
would have indicated that the area was 
actively being ventilated rather than be- 
ing sealed. 

Two unknowns were investigated: (1) the 
performance of the seals in prohibit- 
ing ingassing and outgassing, and (2) the 
potential for the voids inby the seals to 
be directly interconnected, thus enabling 
gas to migrate from one seal to another. 
Since oxygen inflow is directly re- 
lated to seal performance, solving the 
first unknown would enable researchers to 
reach conclusions about the high oxygen 
concentrations . 

Underground sampling took place at six 
different locations (fig. 2). Any SFg 
that escaped from behind the seals would 
be detected at one or possibly two sam- 
pling points {1 or 6). A considerable 
amount of standing water in front of 
seals A-C (fig. 4) made it difficult to 
examine seal A for air leakage. A solu- 
tion to this problem was to connect sam- 
pling tubing to the seal port and extend 
this tubing approximately 500 ft outby 
the seal (from point 2 to point 2'). 
This length of tubing also allowed the 
sampling station to be on the return side 
of the intake air split, eliminating the 
possibility of SF6 reentrainment. To 
sample for SF6 behind seal A, mine per- 
sonnel aspirated the tubing, then began 
sampling gob air. 

Tubing was also used to sample from 
seal A outby (point 3 to point 3') and 
seal D inby (point 5 to point 5') posi- 
tions. This was done to avoid over- 
lapping of SF6 samples at different posi- 
tions. Personal sampling pumps were used 
to continually pull air through the 
flexible tubing in all cases. 

FIRST GOB EVALUATION 

Once SF6 was released behind seal D, 
sampling began. A problem immediately 
surfaced at point 5. The rate at which 
the sampling pump was pulling air sug- 
gested the tubing had a restriction some- 
where in-line. Shortly thereafter, water 
was visible in the flexible tubing. SF6 
samples taken at this position were con- 
sidered unreliable because of the large 
quantity of water that was collecting 




FIGURE 3.— Mine B seals with urethane foam seal outby on roof and ribs. 



inside the tubing. This implied that not 
all of the SFg was injected behind the 
seal. 

Within 30 rain after SFg was released 
behind seal D, 60 pet of the tracer gas 
had already passed sampling point 6. Be- 
cause of this rapid recovery of SF6, it 
appeared that much of the SFg was re- 
leased outby the seal (at point 4) as a 
result of the restriction. 

At sampling points 2, 4, and £, results 
were obtained for approximately 30 min. 
Sampling point 4 , closest to the SFe re- 
lease, showed an extremely high SF6 con- 
centration immediately following the SFg 
injection into seal D. S7(, concentra- 
tions at points 6 and 3 (via 3') peaked 
shortly after point 4. No SF6 was de- 
tected at sample point 2 (via 2'). 

The quantity of SF6 recovered was de- 
termined by the equation: 



QSF 6 = Cx S F 6 TQ air 



(1) 



where Qsf 6 = quantity of SFg recovered 
(ft 3 ) 

Cxsf 6 = mean concentration of SFg 
in one part of air 

T = sampling time (rain) 

and Qair = airflow (ft 3 /min) 

At point 6, considered the most reliable 
sampling position, 0.67 ft 3 or 60 pet of 
the SF6 released was detected within 35 
min. Concentrations at points 3 and 4 
averaged 23 ppb (sampled for 30 min) and 
9,800 ppb (sampled for 65 min), respec- 
tively. The air quantities flowing past 
each sampling point were unknown. No SF6 
was detected at point 1. Since all SF6 
exiting the section had to pass sampling 
point 6, the sum of air quantities at 
points 3 and 4 should have equaled the 




FIGURE 4.— Standing water located outby Mine B seals. 



0.67 ft 3 found in the return. Modifying 
equation 1 slightly enables solving it 
for airflow, rather than for SFg recov- 
ered. The calculated average airflow 
passing the right and left outby posi- 
tions was 1,050 ft 3 /min. 

Because of injection problems, the re- 
sults obtained in this portion of the 
test were limited. However, it was seen 
that no SFg migrated to point 1 or from 
seal D inby (point 5) to seal A inby 
(point 2). 

SECOND GOB EVALUATION 

Because of the obvious sampling prob- 
lems at point 5, researchers examined the 
copper tubing extending through seal D 
into the gob. The inby end of the tube 
was found to be almost totally collapsed 
and under water. The copper tube was re- 
paired and reinserted into seal D, and a 
second bottle of SFg was Injected into 
the seal. Again, air sampling took place 



at all six sampling positions. This 
time, SFg concentrations at point 4 aver- 
aged 320 ppb for 35 min. At point 6, 
mean SFg concentrations were 90 ppb for 
105 sampling minutes. 

After the second release, point 6 
showed only a 29-pct recovery of the 
total SFg injected. Assuming that the 
air quantity previously determined at 
point 4 (1,050 ft/min) was correct, now 
only 4.5 pet of the SF6 released was re- 
covered at that point. Therefore, point 
4 was sampling less gas than the return. 
Since point 4 was within the urethane- 
sealed roof and rib area, SF6 injected 
into the gob was probably leaking around 
the urethane seal and into the return. 

No SF6 was detected at point 2 through- 
out both these tests. Again, any air mi- 
gration from point 5 to point 2 was ei- 
ther extremely slow or nil. 

After the release, the concentration 
at sampling point 5 remained almost con- 
stant (slope 0.02). Knowing this, an 



10 



assumption was made about the relative 
size of the void behind seal D. Mean SFg 
concentration averaged 2.6 vol pet. The 
actual quantity of SF6 released was 1.01 
ft 3 . By proportion, the approximate size 
of the void behind seal D was determined 
to be 39 ft 3 . 

Results of this second phase of testing 
provided more information. Assuming that 
the derived volume behind seal D was cor- 
rect, the likelihood of gas migration 
from one entry to another was very re- 
mote. Therefore, there was little chance 
that SFg released inby seal D would be 
detected in any other gob location. 

GAS ANALYSES OF LEFT AND RIGHT 
INBY POSITIONS 



sampling port during the second gob 
evaluation, which started at 10:30 a.m., 
showed a decrease in O2 and an increase 
in CO2 and the hydrocarbons. Although 
the concentrations of various gases 
linked to gob atmospheres did increase, 
the elevated quantities were still low 
enough to support the belief that air was 
probably leaking through seal D. Since 
hydrocarbon concentrations remained rela- 
tively low, caving behind the seal must 
have been complete and very close to the 
seal. Only a remote connection, due to 
slightly elevated CO2 and CH4, appeared 
to exist between the void and the overall 
gob volume. 

CONCLUSIONS 



In addition to the SFg evaluations, gas 
concentrations behind both seals A and D 
were examined. Again, there appeared to 
be little correlation between data re- 
ceived from the two inby seal positions 
(points 2 and 5, respectively). Seal A 
exhibited characteristics commonly found 
in sealed gobs — depressed O2 and N2 
and elevated quantities of CO2 and 
hydrocarbons (table 4). The air behind 
seal A appeared to be part of the gob 
atmosphere. 

Sampling behind seal D did not produce 
the same results. Initially, O2 samples 
resembled the ambient mine atmosphere. 
Samples obtained through the repaired 



In Mine B, after mining had been com- 
pleted, standard practice was to seal the 
section to eliminate the potential for 
spontaneous combustion. Seals were made 
of pine blocks, hitched into the floor 
and ribs, and coated with spray-applied, 
rigid urethane foam. In addition, to 
reduce the possibility of gob gas escape 
into the working areas of the mine, ex- 
tensive sealing was performed on the roof 
and ribs outby the seals. 

In the particular section examined, 
four seals separated the gob from the 
active workings. The two outside seals 
had sampling ports through which gob gas 
was sampled. 



TABLE 4. 



Gas concentrations behind seal A and seal D, Mine B 



Time 



Concentration, pet 



02 Ar N 2 I CO2 1 CO I CH4 



Concentration, ppm 



C 2 H 6 C 3 H 8 C 4 H 10 C 5 H 12 



SEAL A 



10:15. 
10:55. 
11:30. 
13:00. 
14:30. 



9.0 
8.4 
8.1 
8.0 
8.9 



0.71 
.70 
.70 
.70 
.70 



59.9 
59.0 
58.8 
58.8 
59.7 



8.4 
8.8 
8.9 
8.9 
8.5 



ND 
ND 
ND 
ND 
ND 



21.9 
23.0 
23.4 
23.5 
22.1 



200 
210 
200 
200 
190 



95 

100 

100 

100 

90 



150 
165 
165 
165 
155 



60 
65 
65 
65 

60 



SEAL D 



10:15. 
10:40. 
10:55. 
11:30. 
13:00. 
14:30. 



20.8 
19.3 
19.0 
17.0 
16.6 
18.7 



0.93 
.91 
.89 
.89 
.89 
.91 



78.1 
76.4 
75.0 
74.4 
74.4 
76.1 



0.1 
1.3 

1.8 
2.7 
3.0 
1.8 



ND 
ND 
ND 
ND 
ND 
ND 



ND 
2.1 
3.3 
5.0 
5.1 
2.5 



ND 


ND 


ND 


ND 


15 


9 


15 


7 


20 


12 


22 


9 


10 


10 


25 


10 


10 


8 


20 


10 


20 


5 


12 


6 



ND No gas detected. 



11 



The objective was to determine why gas 
samples taken from the two outside seals 
differed so radically, one exhibiting 
high methane and low oxygen, the other 
just the opposite. 

A sampling strategy was developed. In 
both tests, SF6 was injected inby the 
seal displaying ambient air properties 
(seal D) . The results of these tests in- 
dicated the presence of an extensive 
cave-in just inby that seal. This caving 
was tight enough to separate the area im- 
mediately behind the seal from the rest 



of the gob. Gas analyses showed only a 
remote connection between the space inby 
seal D and the rest of the gob. Samples 
taken inby seal D showed gas concentra- 
tions close to those in ambient air. 
Those samples taken inby seal A were more 
typical of gob gas samples in that they 
had higher concentrations of hydrocarbon 
gases and CO2, and depressed quantities 
of O2. This implied that the area imme- 
diately inby seal D was almost completely 
isolated from the rest of the gob. 



MINE C 



BACKGROUND 

In Mine C, unanticipated gob gas prob- 
lems were occurring behind gob seals that 
separated three different, supposedly 
interconnected, mine sections from the 
working areas of a coal mine. Each sec- 
tion had several seals, one of which had 
a port for gob gas sampling. Section 2 
South exhibited high methane (approx. 50 
pet) , section 3 South had methane concen- 
trations just above the explosive range 
(approx. 17 pet) , and section 4 South ap- 
proximated ambient air. Because of the 
potential danger associated with section 

3 South, the Bureau of Mines was asked to 
perform SF6 ventilation tests on all 
three sections. The objectives were to 
determine seal performance and attempt to 
verify the existence of an air intercon- 
nection between the three sections. 

Mine C is located in southeastern Colo- 
rado. Sections 2 South, 3 South, and 

4 South (fig. 5) are located in a coal 
seam 4 to 11 ft thick, which pitches down 
toward the southwest at a 7-pct slope. 

The main entry (not shown) was driven 
in a westward direction. Several sec- 
tions were developed southward from the 
main. The three sections to be examined 
appeared to be interconnected by an 
east-west development, referred to as 
Panel A. Panel A development took place 

— = — — ^ , . . __ 
'The gob area associated with Panel A 
was where all examinations in this report 
took place. This area will he referred 
to as the gob. 



at the southerly termination of all three 
sections. 

Development, and some retreat pillar 
mining had been completed in the 3 South 
and 4 South sections before they were 
sealed. Approximately 800 ft of the 
southernmost portion of 2 South was also 
sealed. The enclosed gob encompassed 
nearly 6.5 million ft 3 . An active sec- 
tion still existed in the 2 South de- 
velopment, just north of the sealed gob. 
This subsection was being developed west- 
ward and was referred to as Panel B. 

The gob was isolated from the active 
mine by 16-in-thick concrete block seals. 
A ceraentitious sealant was applied to the 
outby and inby faces. Rigid urethane 
foam was spray-applied to the perimeter 
of each seal for added tightness. A 2-in 
ball valve was located in the far left 
and right seals of each section. A 0.25- 
in-OD copper tube for gob gas sampling 
was also located in the left or No. 1 
seal of each section. It was estimated 
that the copper tubes extended into the 
gob for at least 50 ft. 

All three sealed sections were venti- 
lated on return air. An intake split 
from the main entry ventilated Panel B, 
then ventilated the seals across 2 South. 
Both 3 and 4 South seals were ventilated 
by the south leg of the main entry re- 
turns. Brattice-wing curtains, just out- 
by each seal, forced return air to sweep 
the seal face, helping to ensure proper 
seal ventilation. 

To start the evaluation, on separate 
days SF6 was released into the No. 1 seal 



12 



GUUI ILJI Jlj aannqqq^s 






IDDD 

JDDD 

JODD 

IDDD 

□ DD 



4 

South 
seals 



1DD 
IDDi 

1DDI 

IDDi 
)DDI 



DDDQ 

DDDO_ 

DDDDD 

D 

Bl 

G 



Idea 

i c= > <=3 □ □ 



□□and 

\aaDGa 
\aaDQa 

□ DDDC7 

□ DDDD 

□dddd 

□ DDDD 

□dddd 

DDDDD 

□ □□DD 
;□□□□□ 

□ DDDj 



3 South seals 



PANEL B 



IDDI 



] □ □ □□ □□□□□□ ^j-ji. r-, r-. 



2 South seals 
PANEL A 



... ,, v , -DOD, 

bI11 di=, odddd 



DDDDDDni 

ini 



Bl 

a 

Bl 

b 



□DDoa^naa 



□ □DDDdd nlnann^ 



1DDDI 



a 



gggg°°§SDDDDQ 

D gggDDDDDDgD 

[ddddI 

soo 



ocnLzirzi 



Scale, ft 



LEGEND 
Seal 

Wing curtain 
Stopping with door 
Stopping 
Intake air 
Return air 
Caved area 
Gob sample tube 
Return and remote 
gob sampling locations 



□ 

DDDDDD 

□□dddd 

^□DDDD 

UDdddd 
E2 DD 2S 

□DDDDDD 

□□□DDDD 

DQDDDD 




FIGURE 5.— Sealed gob and sampling stations of Mine C. 



of each of the three sections through 
the 0.25-in gas sampling tube. On the 
first day of testing, SFg was injected 
into 4 South; on the second day, into 
3 South; and on the third, into 2 South. 
Immediately following the SF6 release, a 
positive displacement pump, approved by 
MSHA for sampling in methane atmospheres, 
was connected to each tube and run until 
the line was completely purged. This en- 
sured that the SF6 samples would be drawn 
from the gob atmosphere and not from the 
tube itself. 

For these tests, samples were collected 
in two different ways — direct sampling by 
evaluating ambient air in the returns, 



and remote sampling, using a personal 
sampling pump to pull air from inby the 
seals through the gob sampling port. 
Semirigid tubing was connected to the gas 
sampling port that protruded from the 
seal. The sampling pump was connected to 
the outby end of the tubing. 

An in-line plastic tee was placed in 
the tubing at an arbitrary distance be- 
tween the seal and pump. The semirigid 
tubing was connected to two legs of the 
tee. The third leg had an airtight sep- 
tum, through which samples would be ex- 
tracted at previously arranged specific 
time intervals. 



13 



EVALUATION 
4 South 

In the 4 South section, 1.16 ft 3 of SF6 
was released behind the No. 1 seal. The 
seal was ingassing as the SFg was being 
released. The SF6 concentration at this 
location showed little change over the 
entire sampling period. The gas concen- 
trations measured at this point indicated 
a very low rate of diffusion. 

The 4 South return sampling position 
began indicating the presence of SF6 117 
min after release and then continuously 
for the remainder of the test. The quan- 
tity of SFg leaking through the seals 
during the test averaged 0.06 ft 3 /h. 

Thirty minutes after the 4 South return 
began detecting SFg, it was also found in 
the 3 South return. The average SFg flow 
volume at the 3 South return for the test 
was 0.01 ft 3 /h. This yields a dilution 
ratio of 6:1 (from 4 South to 3 South), 
which enabled researchers to determine 
the contribution of 4 South air leakage 
when the 3 South seals were evaluated the 
following day. No SF6 was detected in 
the 3 South gob; thus, it was highly un- 
likely that any SF6 found in the 3 South 
return was leaking through the 3 South 
seals. For the remainder of the test, 
SFg was detected at both the 4 South and 
3 South return positions, with values at 
3 South roughly trailing 4 South peaks by 
20 min. 

3 South 

On the second day, 1.15 ft 3 of SFg was 
released behind the No. 1 seal of the 3 
South section, which outgassed throughout 
the SF6 release. SFg concentrations be- 
hind the No. 1 seal in 3 South were ini- 
tially fairly low and increased through- 
out the sampling period. This was 
contrary to what had been expected, since 
diffusion caused by the kinetic energy of 
the gases at the molecular level should 
tend to dilute the gases and thus reduce 
the concentrations. 

SFg was detected in the 3 South return 
immediately after release. Average SF6 
flow rate through 3 South seals was 
0.04 ft 3 /h. Because of the low SF 6 



concentration detected in the 4 South 
return during this evaluation, all SFg 
sampled in the 3 South return was con- 
sidered to be leaking through 3 South 
seals. If this leak rate had continued 
unchanged, all the SFg in the 3 South gob 
should have been depleted in just under 
26 h. However, as the 3 South section 
approached equilibrium, 8 leakage through 
the seals rapidly diminished. 

Average air leakage from the 3 South 
seals was 1,525 ft 3 /min. If SFg was 
leaking evenly through all six seals, the 
average air leakage was slightly more 
than 250 ft 3 /min per seal. 

Remote gob samples taken behind the 4 
South seals showed that SFg concen- 
trations decreased by a factor of 1,500 
over a 24-h period. This large reduction 
took place even though very little con- 
centration change was noted throughout 
the second day's sampling time. 

2 South 

On the third day, 1.13 ft 3 of SF 6 was 
released behind the 2 South seal. This 
seal also was outgassing during the SFg 
release. Again, the average SFg concen- 
tration began at a low value and in- 
creased throughout the test. Assuming 
that the highest concentration attained 
signified equilibrium in the gob, the 2 
South gob appeared to equilibrate much 
faster than the 3 South gob. 

As in the 3 South test, SFg was de- 
tected in the 2 South return immediately 
after release. Return concentrations 
remained high throughout the test. Aver- 
age SFg flow rate through the seals was 
0.18 ft 3 /h. At this rate, SFg was pro- 
jected to fully deplete in slightly over 
6 h. Again, as the gob air began to ap- 
proach equilibrium, SFg flow through the 
seals diminished rapidly. 

The air leakage through the seals was 
calculated by comparing the concen- 
trations behind and in front of the 

^Equilibrium in this context means the 
sampling time in which SFg concentrations 
fall, then maintain a constant value, 
with no measurable additional diffusion 
taking place. 



14 



seals. If the SFg was leaking through 
only the one seal with the sampling port, 
leakage was 3,569 ft 3 /min. If all seven 
seals were leaking, the average airflow 
was 509 ft 3 /min. 

Remote gob samples taken inby the 3 
South seals were surprising, in that the 
average SF6 concentration increased, 
rather than decreased, compared with 
levels from samples taken the previous 
day. In fact, the values were almost 
twice as high as the day before. At the 
same time, remote gob samples also showed 
that the concentration in the 4 South gob 
continued to decrease very slowly. In 
both cases, the concentration changed 
very little during sampling, showing 
little additional gas dilution throughout 
the gob. 

CONCLUSIONS 

4 South 

Because the SFg concentration behind 
the 4 South seal did not decline mea- 
surably throughout the first-day test, 
the gas was apparently being released 
into a problem area, probably into some 
tightly caved or blind area where little 
mixing was able to take place. However, 
since SF6 leakage did occur through the 
seals, the air in the gob was moving. 
The sample probe, therefore, may have 
been poorly located and may not have in- 
dicated what was actually happening in 
the gob. 

Following the first day of sampling, 
only very small quantities of SFg were 
detected in the 4 South returns. During 
initial SF6 release, the seal was ingas- 
sing, but it began outgassing during the 
next two sampling days. It was probable 
that, during hours when no SF6 sampling 
was taking place, more SF6 outgassed into 
the return. Mine conditions did not ap- 
pear to be sufficiently severe to cause 
pressure changes that would have created 
an appreciable change in the mine 
atmosphere. 

Twenty-four hours after the SFg release 
in 4 South, samples detected no SFg in 
the 3 South gob. Even after more than 48 
h, the 2 South gob sampling position 
still had not detected any SF6 from 



4 South. Based on this, there ap- 
peared to have been no low-resistance air 
interconnection between sections. 

The seals themselves appeared ade- 
quately constructed to preclude large- 
scale gob gas leakage. However, 
a periodic visual inspection was 
recommended to ensure that the seals, and 
especially their mortar sealants, re- 
mained solid. 

3 South 

It was not determined why, after SF6 
was released behind the 3 South seal, the 
concentration increased throughout the 
sampling period. According to gas dif- 
fusion laws, because of the random motion 
of molecules and their associated kinetic 
energy, the net flow of gas is from high 
concentrations to low. One explanation 
proposed was that if the sampling probe 
was at the roof and the gob atmosphere 
was static, the SFs, which is more than 4 
times as dense as air, would require a 
long time to dilute back to the probe. 
However, this would be valid only if 
there was little air movement in the gob; 
the immediate detection of SF6 in the 3 
South return suggests air movement behind 
the seals. 

During the 2 South evaluation, SF6 con- 
centrations in the 3 South gob were 
stable but approximately twice as high as 
the initial peak when SFg was released 
into 3 South. SF6 continued to increase 
from the time gas was released into the 
section until equilibrium was estab- 
lished. Since no SF6 change occurred in 

3 South during sampling the day after 
SF6 release in 3 South, the gob was 
considered to be at equilibrium. 

Again, an open interconnection between 

4 South and 3 South did not appear to 
exist. No SF6 was detected in 3 South 
even as late as 24 h after its release in 
4 South. The distance from the release 
point in 4 South to the sample point in 3 
South was approximately 1,300 ft. Thus, 
a diffusion rate of less than 1 ft/min 
was required to detect S¥(, in the 3 South 
gob. Still, no SFg was found. 

Initially, it was thought that the in- 
crease in SF6 concentration 24 h after 
release into 3 South was due to the 



15 



mixing of gas in 3 South and 4 South 
sections. No reasonable explanation 
could be given for the large difference 
in concentrations between the two sec- 
tions. It seemed logical that, if the 
sections were readily interconnected, 
equilibrium would have provided a common 
SF6 value for both locations. 

The seals associated with the 3 South 
section also appeared to be well con- 
structed. During testing, their leakage 
values approximated what was expected 
from most concrete block seals. Although 
appreciable quantities of SF6 existed 
behind the seals, only minute quantities 
were sampled in the return. 

2 South 

As in the 3 South gob, SF6 released 
into 2 South gob also exhibited char- 
acteristics that cannot be readily 
explained by gas diffusion laws. The SF6 
concentrations behind the seal appeared 
to stabilize near the end of the test. 
This equilibration occurred much faster 
in 2 South than in other sections. The 



inference here was that the unobstructed 
volume behind 2 South appeared to be less 
than the volume behind 3 South seals. 
Looking at the mine map, this theory 
might be valid if all stoppings shown in 
2 South were still in place. 

No SFg was detected in the 2 South gob 
prior to gas release in that section. 
Since the first release of SFg was 48 h 
earlier, it was evident that no readily 
accessible passageway existed between the 
4 South and 2 South sections. The aver- 
age air velocity required for transmis- 
sion of SFe from 4 South to 2 South with- 
in 48 h would have been slightly more 
than 1 ft/min. 

Section 2 South was apparently not as 
well sealed as the other two sections, as 
proven by the higher average air leakage 
through the seals. At several places 
across the seal face, small areas of high 
(greater than 10 pet) methane concen- 
trations were detected. These data bore 
out suppositions based on prior visual 
inspection. Recoating these seals with a 
sealant would probably have eliminated 
the additional leakage. 



SUMMARY 



Two options, ventilating or sealing, 
are available to mines having abandoned 
areas. If the gob is to be ventilated 
after abandonment, there should be sev- 
eral bleeder entries around the gob, as 
well as a pressure differential across 
the gob, to ensure adequate air movement. 
To further enhance gob ventilation, at 
least one borehole should be driven into 
the section before or after mining. 

Gobs that liberate excessive quantities 
of methane, or are prone to spontaneous 
combustion, are usually sealed (2). 
Seals must be constructed of substantial, 
incombustible material and must be 
able to arrest an explosion. A sec- 
tion that is to be sealed after min- 
ing should have a minimum number of 
entries, consistent with good health 
and safety practices. Seals are not 
100 pet efficient; therefore, minimiz- 
ing their number reduces the poten- 
tial for gob leakage. Boreholes also 



enhance the performance of sealed gobs. 
These are developed vertically from 
the surface, or horizontally through a 
seal to the return. 

Each mine examined was faced with a 
different situation that required spe- 
cific information to determine gob per- 
formance. All of the research was per- 
formed using sulfur hexafluoride (SFs) 
gas as the primary air tracer. Mine A 
ventilated its gobs; research determined 
both the average gob air velocity and the 
locations around the gob that were more 
thoroughly ventilated. Mine B sealed its 
gobs, but was receiving conflicting gas 
sampling results from two seals isolating 
the same gob; research found a restric- 
tion behind one seal that altered sampled 
gas concentrations. Mine C has a large 
gob, containing three sections with a 
total of 18 seals. This gob was experi- 
encing different methane concentrations 
in each section; results showed that the 



16 



individual sections were not inter- 
connected. Average leak rates through 



the seals in each section were 
determined. 



also 



REFERENCES 



1. American Conference on Governmental 
Industrial Hygienists. Threshold Limit 
Values for Chemical Substances and Phys- 
ical Agents in the Work Environment With 
Intended Changes for 1983-84. Cincin- 
nati, OH, 1983, 93 pp. 

2. Foster-Miller, Inc. Improved Ven- 
tilation of Sealed Gobs - Phase I Re- 
port. Ongoing BuMines contract JO308029; 
for inf., contact R. J. Tirako, TPO, 
Div. Health and Saf. Technol. , BuMines, 
Pittsburgh, PA. 

3. Thimons, E. D. , R. J. Bielicki, and 
F. N. Kissell. Using Sulfur Hexafluoride 



as a Gaseous Tracer To Study Ventilation 
Systems in Mines. BuMines RI 7916, 1974, 
22 pp. 

4. Timko, R. J., and E. D. Thimons. 
Sulfur Hexafluoride as a Mine Ventilation 
Research Tool — Recent Field Applications. 
BuMines RI 8735, 1982, 15 pp. 

5. U.S. Code of Federal Regulations. 
Title 30 — Mineral Resources; Chapter I — 
Mine Safety and Health Administration, 
Dep. Labor; Subchapter — Coal Mine Safe- 
ty and Health; Part 75 — Mandatory Safety 
Standards-Underground Coal Mines; July 1, 
1981, pp. 451-566. 



C 24 4 



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